1. Field of the Invention
The present invention relates generally to the field of cathodic corrosion protection systems. More specifically, the present invention discloses a variable current solar powered cathodic corrosion protection system in which a battery and a solid state voltage regulator are used to provide a minimum current that is a predetermined fraction of the nominal constant current generated by an equivalent conventional power supply.
2. Statement of the Problem
Cathodic protection systems have been widely used for many years to prevent corrosion in pipelines, well casings, and other metallic bodies that are buried in an electrolyte. In a conventional cathodic protection system, the metallic body to be protected is connected to the negative terminal (or cathode) of a current source. The positive terminal of the current source is connected to a conductive ground bed (or anode). The current source induces a current from the metallic body (-) to the ground bed (+) through the ground. The resulting negative polarization of the metallic body tends to minimize corrosion. The traditional approach has been to maintain a relatively constant current over time. In cases where an AC power supply is readily available, this is accomplished simply by means of a transformer and rectifier to produce a constant DC current of the desired amplitude.
However, even in cases where an AC power supply is not available, the conventional approach has largely been to substitute a power supply (e.g., photovoltaic panels and batteries) that are designed to produce a relatively constant current over each 24 hour period. This simplistic approach is wasteful in that it requires unnecessarily large photovoltaic panels and excessive battery capacity. Instead, studies have shown that once the metallic body has been initially polarized, a minimal current is usually sufficient to maintain polarization for an extended period of time thereafter. Thus, it is possible to design a variable current cathodic protection system having a peak photovoltaic capacity sufficient to polarize the metallic body, but with a backup system (e.g., batteries) for nighttime operation that provides a fraction of the nominal constant current provided by a conventional power supply. For example, assuming the nighttime (or low cycle) current is reduced to 30% of the nominal current, the required capacity of the solar panels can be reduced by approximately 45% and the required battery capacity can be reduced by approximately 90%.
A number of photovoltaic cathodic protection systems have been invented in the past that provide reduced current during nighttime, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Carpenter, et al. 5,026,468 June 25, 1991 Carpenter, et al. 5,139,634 Aug. 18, 1992 ______________________________________
"Solo Photovoltaic Power Supplies for Cathodic Protection" brochure by Burge Corrosion Systems, Aztec, N. Mex. PA0 SUN-LOC brochure by Guardian Corrosion Control Corp., Montrose, Colo. PA0 SUN-GUARD brochure by Guardian Corrosion Control Corp., Montrose, Colo.
The patents of Carpenter, et al. disclose a dual ground bed cathodic protection system that uses a solar power supply to generate current during daytime and a sacrificial anode to generate current at night. The system includes automatic controls to switch between the solar power supply and the sacrificial anode.
The brochure by Burge Corrosion Systems discusses a cathodic protection system powered by photovoltaic panels with a nickel-cadmium battery backup. For example, a nominal 6-volt/6-amp photovoltaic array is allowed to reach 8-volt/8-amp output levels at peak insolation during the day. After dark, the Ni-Cad batteries provide a minimum of 4-volt/4-amp with no sun. The brochure notes that studies have shown that adequate polarization on well casings can be retained for protection over long periods of time with as little as 10% of the current required for initial polarization.
The SUN-LOC brochure by Guardian Corrosion Control Corp. discusses several cathodic protection systems that are powered by photovoltaic panels. The simplest version shown in FIG. 3 of this brochure has no battery backup. The traditional approach shown in FIG. 1 of the brochure uses lead-acid batteries to provide essentially constant current. The embodiment shown in FIG. 2 of the brochure uses lead-acid batteries to provide approximately 50% of nominal current at night. Lead-acid batteries require a fairly sophisticated voltage regulation system to prevent both overcharging and excessive discharging that can damage lead-acid batteries.
The SUN-GUARD brochure discusses the corresponding newer models of the cathodic protection systems produced by Guardian Corrosion Control Corp. using nickel-cadmium batteries in place of lead-acid batteries. The versions of these systems that were sold more than one year prior to the filing date of this application used zener diodes (and other discrete components) to control the voltage across the battery, and did not use a solid state voltage regulator.
3. Solution to the Problem
None of the prior art references uncovered in the search show a variable current photovoltaic cathodic protection system that uses a battery and a solid state voltage regulator to provide a minimum current that is a predetermined fraction of the nominal current generated by conventional power supply.